1. Field of the Invention
The present invention relates to disk playback devices for playing back optical disks or magnetic disks, and more particularly, to decoders and decoding methods for adaptively equalizing partial response (PR) of playback waveforms and for performing decoding using the adaptively equalized partial response.
2. Description of the Related Art
A fixed delay tree search (FDTS) has been known as a decoding technique. By using the FDTS, hard-decision decoding that does not require a large delay in decoding in a path memory, unlike Viterbi decoding, and that is capable of performing decoding with a fixed delay is available (see, for example, Jeakyun Moon, “Performance Comparison of Detection Methods in Magnetic Recording”, IEEE Transaction on magnetics, Vol. 26, No. 6). In contrast, soft-decision decoding, which is known as partial response maximum likelihood (PRML), for Viterbi-decoding a waveform after PR equalization is also available (see, for example, Roy D. Cideciyan, “A PRML System for Digital Magnetic Recording”, IEEE Journal on Selected Areas in Communications, Vol. 10, No. 1, January 1992).
A decoding delay when the above-mentioned playback waveform is decoded will be considered. Generally, in Viterbi decoding, several tens of clocks are required for merging paths. Thus, several tens of taps are required for path memories, and a decoding delay of several tens of clocks is required. The FDTS depends on the length of a tree structure (hereinafter, referred to as a “tree length”). Generally, the FDTS requires only a decoding delay of several clocks, which substantially corresponds to an inter-symbol interference (ISI) length of a PR.
In general control loops, such as PLLs, when a delay within a loop (hereinafter, referred to as a “loop delay”) is large, a phase margin and a gain margin are reduced and the loop enters an unstable state. Thus, zero-crossing detection with less delay in tentative determination for error detection is used. However, when such zero-crossing detection is used for a communication channel with large noise, high-density magnetic recording in which a high-pass characteristic is considerably attenuated, or high-density optical recording in which an output is not available after cutting off a magnitude transfer function (MTF), the bit error rate (BER) of a data detection result increases. Thus, detection with a very low performance is performed, and error detection calculation cannot be performed accurately.
In error detection of general control loops, using data realizing reasonable detection performance with a small loop delay is more desirable than using data realizing excellent detection performance with a large loop delay. In view of this, as error detection under conditions that a zero-crossing detection result is invalid but that failures in the FDTS and Viterbi decoding are prevented, using the FDTS as a detection method for ensuring a phase margin and a gain margin of a control loop is advantageous.